Ring Opening Polymerization and Copolymerization of Cyclic Esters Catalyzed by Group 2 Metal Complexes Supported by Functionalized P-N Ligands

Article

pubs.acs.org/IC

Cite This: Inorg. Chem. XXXX, XXX, XXX −XXX

Ring Opening Polymerization and Copolymerization of Cyclic Esters

Catalyzed by Group 2 Metal Complexes Supported by Functionalized

P−N Ligands

Adimulam Harinath,^†Jayeeta Bhattacharjee,^†Alok Sarkar,^‡Hari Pada Nayek,^§and Tarun K. Panda*^,†

^†Department of Chemistry, Indian Institute of Technology Hyderabad Kandi 502 285, Sangareddy, Telangana, India

^‡Momentive Performance Materials Pvt. Ltd., Bangalore 560 100, India

^§Department of Applied Chemistry, Indian Institute of Technology (ISM), Dhanbad 826 004, Jharkhand, India

S

* Supporting Information

ABSTRACT: We report the preparation of alkali and alkaline

earth (Ae) metal complexes supported by 2-picolylamino-

diphenylphosphane chalcogenide [(Ph₂P(=E)NHCH₂-

(C₅H₄N)] [E = S (1-H); Se (2-H)] ligands. The treatment

of the protic ligand, 1-H or 2-H, with alkali metal

hexamethyldisilazides at room temperature aﬀorded the

corresponding alkali metal salts [M(THF)₂(Ph₂P(=E)NCH₂-

(C₅H₄N)] [M = Li, E = S (3a), Se (3b)] and [{M(THF)_n-

(Ph₂P(=E)NCH₂(C₅H₄N)}₂] [M = Na, E = S (4a), Se (4b);

M = K, E = Se (5b)] in good yield. The homoleptic Ae metal

complexes [κ²-(Ph₂P(=Se)NCH₂(C₅H₄N)Mg(THF)] (6b)

and [κ³-{(Ph₂P(=Se)NCH₂(C₅H₄N)}₂M(THF)_n] (M = Ca

(7b), Sr (8b), Ba (9b)] were synthesized by the one-pot reaction of 2-H with [KN(SiMe₃)₂] and MI₂in a 2:2:1 molar ratio at

room temperature. The molecular structures of the protic-ligands 1-H and 2-H, as well as complexes 3a,b−5a,b and 6b−9b were

established using single-crystal X-ray analysis. The Ae metal complexes 6b−9b were tested for ring-opening polymerization

(ROP) of racemic lactide (rac-LA) and copolymerization of rac-LA and ε-caprolactone (ε-CL) at room temperature. In the ROP

of rac-LA, the calcium complex 7b exhibited high isoselectivity, with P_i= 0.89, whereas both the barium and strontium complexes

showed lower isoselectivity with P_i= 0.78−0.62. In the copolymerization of rac-LA and ε-CL, both barium and strontium

complexes proved to be eﬃcient precatalysts for the formation of the block copolymer rac-LA-CL, but the reactivity of 9b was

found to be better than that of 8b. All the polymers were fully characterized using diﬀerential scanning calorimetry,

thermogravimetric analysis, and gel permeation chromatography analyses. Kinetic studies on the ROP reaction of LA conﬁrmed

that the rate of polymerization followed the order Ba ≫ Sr ≈ Ca.

Hitherto, catalytic ROP of cyclic esters, such as LAs and

lactones, has been explored by the use of organocatalysts,¹²⁻¹⁴

as well as discrete metal precursors of chiral ligands, such as

complexes of tin,^15,16aluminum,¹⁷⁻¹⁹zinc,^20,21magnesium,^22,23

iron,^24,25titanium,^26,27indium,²⁸yttrium,²⁹rare-earth met-

als,^30,31and organo-initiators.³²Generally, only a limited

number of isoselective achiral catalysts are well-known in the

literature (Figure 1),³³and most isoselective ROPs are based

on expensive chiral catalysts. Several highly active achiral

catalysts³⁴are well-known for producing amorphous, hetero-

tactic PLAs of low PDI from rac-LA.³⁵⁻⁴⁴However, it is highly

desirable to develop a new achiral catalyst⁴⁵⁻⁴⁷that can

promote stereoselective transformations by adopting chirality in

the propagating step by being chain-end bound to the catalyst

in a chain-end controlled mechanism. This strategy is

INTRODUCTION

■

Recently, polylactide (PLA), a biodegradable and biocompat-

ible polymer, has garnered much interest as an excellent

replacement for conventional petrochemical materials in a

variety of applications, including packaging, ﬁbers, and

biomedical devices.¹⁻⁴PLA is produced from the ring-opening

polymerization (ROP) of lactide (LA) monomers derived from

renewable resources, which are nontoxic.⁵PLAs with diﬀerent

microstructures can be synthesized by varying diﬀerent isomers

of the LA monomer, selectivity of initiators, and polymerization

conditions. In addition, PLA has unique physical properties that

render it useful in a wide range of commodity applications as

well as in life sciences.^6,7The ROP of racemic lactide (rac-LA)

may produce either (a) an isotactic PLA or (b) a heterotactic

PLA and atactic PLA. Isotactic PLAs obtained from the ROP of

rac-LA have higher melting points (T_m) and modulus than

atactic PLAs. In this context, there is increasing interest in

methods that allow for the preparation of isotactic PLAs in a

reproducible and controlled fashion.⁸⁻¹¹

Received: November 8, 2017

A

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Figure 1. Type of achiral catalysts.

advantageous, as it prevents the need for regenerated chiral

precursors that control the stereochemistry in rac-LA.⁴⁸⁻⁵¹

In certain medicinal applications requiring advanced drug-

delivery materials with precise control over the polymer

degradation rate, the use of homopolymers is often detrimental,

due to their rapid biodegradability and impermeability to most

drug molecules. It has been shown that the copolymerization of

LA with ε-caprolactone (ε-CL) can lead to variety of

biodegradable copolymers with improved properties, when

compared to their parent polymers, polycaprolactone (PCL)

and PLA homopolymers.^52,53PLA obtained from rac-LA is

typically characterized as brittle, with low strength and short in

vivo lifespan (typically a few weeks), whereas PCL is known to

be tough, high in mechanical strength, and has a lifespan of

about a few years. Therefore, combining these two materials

into a random copolymer of varying compositions has been a

very useful technique in producing biomaterials tailored for

speciﬁc applications. The ring-opening copolymerizations of LA

and caprolactone (CL) are generally carried out using stannous

octoate, zinc alkoxide, aluminum isopropoxide, or titanium

isopropoxide as the catalyst.⁵⁴⁻⁶⁰However, due to the relatively

high reactivity of LA when compared to CL, this reaction often

leads to the formation of copolymers with blocky structures.⁶¹

Yttrium amidate complexes have been shown to produce PCL

of high molecular weight.⁶²However, the moisture sensitivity of

this catalyst limits its application toward achieving scalable,

functionalized polymer synthesis.

homopolymerization,⁶⁵⁻⁶⁹and the use of nontoxic heavier Ae

metal catalysts is much less explored albeit some magnesium

complexes are known.^22,23

Recently, we employed a series of Ae metal complexes as

catalysts in the ROP of rac-LA and ε-CL at room temper-

ature.⁷⁰Despite high selectivity toward LA polymerization,

these complexes failed to act as suitable catalysts for the

copolymerization of rac-LA and ε-CL. To explore the further

activity of Ae metal complexes in more diverse polymerizations,

we introduced a series of Ae metal complexes with function-

alized P−N ligands for the homopolymerization of rac-LA and

ε-CL, followed by copolymerization of rac-LA and ε-CL. All the

complexes showed high activity, with high isoselectivity, and

controlled polymerization at high molecular weights for the

ROP of rac-LA and ε-CL. Most importantly, the Ae metal

complexes can be used for copolymer synthesis with short

sequence lengths, close to 1:1 monomer incorporation, and

good M_ndue to transesteriﬁcation. We intend to further

explore our synthesis of random copolymers with these readily

modiﬁed ligand sets to access a family of polymerization

catalysts which will aﬀord a large scope of M_n, with shorter

sequence length and increased levels of transesteriﬁcation.

RESULTS AND DISCUSSION

■

Ligand Synthesis. The ligand precursors, 2-picolylamino-

diphenylphosphane chalcogenide [(Ph₂P(=E)NH−CH₂-

(C₅H₄N)] (E = S (1-H); Se (2-H), were readily obtained in

excellent yield and high purity by the equimolar oxidation of 2-

picolylamino-diphenylphosphane using a slight excess of

elemental sulfur or selenium in toluene at 90 °C temperature

(Scheme 1). The ¹H NMR spectra of both the ligands 1-H and

2-H are consistent with their composition. In their ³¹P{¹H}

Indeed, there are very few examples of random copolymer

formation using designed catalysts, and only a few from

titanium and rare-earth metal-based catalysts have been

reported.^63,64There are even fewer reports of the use of

sodium and potassium complexes as precatalysts in the case of

B

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sodium, and potassium ions, respectively. The details of the

structural parameters are given in Table TS1 in the Supporting

Information. Both the lithium complexes are monomeric,

whereas the sodium and potassium complexes are dimeric in

nature. The solid-state structures of complexes 3a, 3b, 4a, 4b,

and 5b are shown in Figures FS3, FS4, FS5, FS6, and FS7 in

Supporting Information, respectively.

Scheme 1. Synthesis of protic ligands 1-H and 2-H

Ae Metal Complexes. Using a known strategy,⁷⁸⁻⁸⁰the

one-pot reaction of 2-H with [KN(SiMe₃)₂] in THF, followed

by the addition of AeI₂(in 2:2:1 molar ratio), yielded the

homoleptic complexes [κ²-{(Ph₂P-(=Se)NCH₂(C₅H₄N)}₂Mg-

(THF)] (6b) and [κ³-{(Ph₂P(=Se)NCH₂(C₅H₄N)}₂M-

(THF)_n] (M = Ca (7b), Sr (8b), Ba (9b)] in good quantities

(Scheme 3). The newly synthesized Ae complexes 6b, 7b, 8b,

and 9b were characterized using spectroscopic and analytical

techniques. The solid-state structures of complexes 6b and 9b

were conﬁrmed by single-crystal X-ray diﬀraction analysis.

The Ae metal complexes are readily soluble in toluene and

THF. A singlet resonance was observed for each complex in the

³¹P{¹H} NMR spectra measured in C₆D₆at δ_P70.6 (for 6b),

71.0 (for 7b), 70.9 (8b), and 70.9 (9b) ppm, along with two

satellite peaks due to the coupling of P−Se with the adjacent

selenium atom. These chemical shift values are signiﬁcantly

shifted toward the lower ﬁeld compared to that of free ligand 2-

NMR spectra, similar resonances at δ_P= 59.7 (for 1-H) and

57.6 ppm (for 2-H) are consistent with those of related

compounds [(Ph₂P(=S)NH−CO(C₅H₄N)] (δ_P= 54.4 ppm)

and [(Ph₂P(=Se)NH−CO(C₅H₄N)] (δ_P= 47.8 ppm).⁷¹

The solid-state structure of 1-H and 2-H is shown in Figures

FS1 and FS2 in Supporting Information. The P−E bond

distances [1.9553(7) for 1-H and 2.1061(7) Å for 2-H] are in

good agreement with those of previously reported compounds

[Ph₂P(S)NHCPh₃] [1.9472(7) Å] and [Ph₂P(Se)NH(2,6-

Me₂C₆H₃)]⁷²⁻⁷⁷[2.1019(8) Å], and thus can be diagnosed

as P = S and P = Se double bonds. The P−N bond distance of

1.662(2) Å is consistent with those measured in other

phosphinamines.⁷²⁻⁷⁷

1

H (δ_p= 57.6 ppm). In the H NMR spectra of each complex,

the absence of a resonance signal at 4.03 ppm (assigned for −

NH proton of ligand 2-H) conﬁrms the formation of a

monoanionic fragment of ligand 2. A doublet signal of each

complex was observed at δ_H4.26 (for 6b), 4.16 (for 7b), 4.26

(for 8b), and 4.16 (9b) ppm due to the resonance of methylene

protons present in the 2-picolylamino-diphenylphosphane

selenide ligand backbone.

Alkali Metal Complexes. Alkali metal complexes, with

molecular formulas [M(THF)₂(Ph₂P(=E)NCH₂(C₅H₄N)] [M

= Li, E = S (3a), Se (3b)] and [{M(THF)_n(Ph₂P(=E)-

NCH₂(C₅H₄N)}₂] [M = Na E = S(4a), Se (4b); M = K, E = Se

(5b)], were prepared by the reaction between 1-H or 2-H and

[MN(SiMe₃)₂] (M = Li, Na, K) in THF (Scheme 2). All ﬁve

There is also considerable interest in Ae organometallics, in

particular, the non-cyclopentadienyl-based ligand system,⁸¹⁻⁸⁴

and complexes 6b−9b represent, to the best of our knowledge,

the ﬁrst examples of Ae metal complexes with the 2-

picolylamino-diphenylphosphane selenide ligand backbone.

Therefore, their molecular structures in the solid state were

determined using X-ray diﬀraction analysis. The crystals of

complexes 7b and 8b diﬀract poorly. However, the molecular

structures of magnesium complex 6b and barium complex 9b

were determined using single-crystal diﬀraction analysis. The

details of the structural parameters are given in Table TS1 in

the Supporting Information. The solid-state structures of

complexes 6b and 9b are shown in Figures 2 and 3,

respectively. In the magnesium complex 6b, a κ²-coordination

mode of the ligand was observed, leaving the selenium atom

without any coordination. In addition, a coordinated THF

molecule takes the formal coordination number around the

magnesium ion to ﬁve. Thus, the arrangement around the

magnesium ion is an intermediate structure, between a square-

based pyramidal and trigonal bipyramidal geometry. However,

with a geometric parameter of τ₅= 0.69,⁸²the metal ion can be

best described as having a highly distorted trigonal bipyramidal

geometry around it, with N2, N4, and O1 atoms in equatorial

positions, and N1 and N3 atoms in axial positions. Two

planesdeﬁned by Mg1, N4, C12, C7, N3, and N1, Mg1, N2,

C6, C5are non-coplanar, as indicated by the dihedral angle of

59.8° between them. We noted that in complex 6b, both the

pyridine nitrogen and amido-nitrogen atoms maintain a trans

arrangement to each other through the magnesium ion.

In the molecular structure of barium complex 9b, both the

ligand fragments exhibited the κ³coordination mode using

Scheme 2. Synthesis of Alkali Metal Complexes 3a, 3b, 4a,

4b, and 5b

complexes were characterized using spectroscopic and

analytical techniques, and the solid-state structures of

complexes 3a, 3b, 4a, 4b, and 5b were conﬁrmed by single-

crystal X-ray diﬀraction analysis.

The molecular structures of 3a and 4a conﬁrm the

attachment of the sulﬁde ligand 1 to lithium and sodium ions

respectively, whereas the solid-state structures of 3b, 4b, and 5b

reveal the bonding between the selenide ligand 2 and lithium,

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Scheme 3. Synthesis of Ae Metal Complexes (6b−9b)

Figure 3. Molecular solid-state structure of [κ³-{(Ph₂P(=Se)NCH₂-

(C₅H₄N)}₂Ba(THF)₂] (9b). Selected bond lengths (Å) and angles

(deg) are Ba1−N1 2.858(8), Ba1−N2 2.740(8), Ba1−Se1 3.4185(11),

Ba1−N3 2.875(7), Ba1−N4 2.760(7), Ba1−Se2 3.4161(12), Ba1−O1

2.825(7), Ba1−O2 2.789(7), N1−Ba1−N2 58.4(2), N1−Ba1−Se1

116.17(15), N1−Ba1−N3 99.0(2), N1−Ba1−N4 82.5(2), N1−Ba1−

Se2 88.66(16), N2−Ba1−Se1 58.45(14), N3−Ba1−N4 58.0(2), N3−

Ba1−Se2 114.41(15), N4−Ba1−Se2 58.85(15), O1−Ba1−O2

94.1(2), N2−P1−Se1 109.1(3), S1−Ba1−Se2 147.20(3).

Figure 2. Molecular solid-state structure of [κ²-(Ph₂P(=Se)NCH₂-

(C₅H₄N)Mg(THF)] (6b). Selected bond lengths (Å) and angles

(deg) are Mg1−N1 2.159(5), Mg1−N2 2.076(5), Mg1−N3 2.143(5),

Mg1−N4 2.056(5), Mg1−O1 2.107(5), P1−Se1 2.1377(17), P2−Se2

2.1397(18), N1−Mg1−N2 79.4(2), N1−Mg1−N3 163.7(2), N1−

Mg1−N4 110.6(2), N2−Mg1−N3 105.0(2), N2−Mg1−N4 122.0(2),

N3−Mg1−N4 80.7(2), N1−Mg1−O1 82.9(2), N2−Mg1−O1

130.1(2), N3−Mg1−O1 82.4(2), N4−Mg1−O1 107.9(2).

achieved after 24 h (Figure FS60 in Supporting Information).

So further polymerization reactions were carried out using

other alkaline earth metal catalysts. The polymerization of rac-

LA, catalyzed by complexes 7b and 8b, was accomplished with

99% conversion within 30 min [100:1 ratio of [rac-LA]:[7b]/

[rac-LA]:[8b] in toluene (Table 1, entries 1 and 6), [Cat.] =

1.0 mM, room temperature]. In contrast, when the

corresponding barium complex 9b was used as the catalyst,

the reaction was much faster, and 99% conversion was achieved

in 5 min (Table 1, entry 11). However, the same reaction, using

9b as the catalyst, took 3 h to achieve 50% monomer

conversion when THF was used as solvent and 25% when

CHCl₃was used (Table 1, entries 17 and 18). The varying

reaction rates can be ascribed to the competitive coordination

of THF and diﬀerent solvent polarities with active metal

centers, which diminish the LA−metal interaction. We have

observed and reported similar phenomena previously.⁶⁹

Therefore, toluene was chosen as an optimized solvent to

eliminate solvent inﬂuence in the ROP of LA. When we

pyridyl, amido nitrogen, and selenium atoms to coordinate with

the barium ion. The barium center is 8-fold coordinated, owing

to ligation from the additional two THF molecules. Thus, the

barium ion can be best described as having a distorted square

antiprismatic geometry around it. Each ligand fragment formed

one ﬁve-membered (Ba1−N1−C5−C6−N2 or Ba1−N3−

C23−C24−N4) and one four-membered (Ba1−N2−P1−Se1

or Ba1−N4−P2−Se2) metallacycle, fused together in the metal

coordination polyhedron. We noted that in complex 9b, both

ligands were bonded to the metal ion in a cis-orientation, unlike

the magnesium complex 6b. The Ba−Se distances [3.4185(1)

and 3.4161(1) Å] are consistent with those in the complex we

have previously reported [{Ph₂P(Se)N}₂C₆H₄]Ba(THF)₃

(3.4706(9) and 3.4071(9)Å].

ROP of rac-LA. All the complexes (6b−9b) were tested as

catalysts for the ROP reaction of rac-LA and values are shown

in Table 1. In the case of magnesium complex 6b the rate of

polymerization is very slow, and only 47% conversion was

D

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Table 1. Polymerization of rac-LA in the Presence of Alkaline Earth Metal Complexes Bearing Phosphinamine Selenide Ligand

a

(7b−9b)

b

c

d

entry

catalyst

M:1

solvent

time (h:m)

conversion

M_ntheo(kDa)

M_nexpiGPC (kDa)

PDI

P_i

1

7b

8b

9b

7b

100

200

300

400

500

100

200

300

400

500

100

200

300

400

500

1000

100

200

300

toluene

THF

00.30

00.35

00.40

01.00

00.30

00.35

00.40

00.05

00.06

00.08

00.10

03.00

00.30

99

95

97

99

97

99

95

99

91

50

25

99

14.3

28.5

41.1

55.9

71.3

14.3

28.5

41.1

54.7

69.8

14.3

28.5

42.8

57.1

71.3

131.04

7.2

15.9

29.9

42.3

57

1.24

1.23

1.27

1.04

1.64

1.35

1.57

1.38

1.50

1.12

1.35

1.31

1.40

1.52

1.01

1.51

1.35

1.75

1.23

1.24

0.89

0.87

0.85

0.83

0.85

0.79

0.77

0.76

0.75

0.71

0.70

0.58

0.65

0.62

0.60

0.61

0.55

0.86

2

3

4

5

74.1

15.9

29.9

40.6

54.9

68.2

14.6

29.5

45.9

55.8

73.9

125.4

5.6

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

DCM

3.6

2.4

toluene

28.5

41.1

27.5

42.9

e

toluene

a

b

In toluene at 25 °C, [catalyst] = 1 mM. Conversions were determined by ¹H NMR spectroscopy. M_ntheo= molecular weight of chain-end + 144 g

c

mol⁻¹× (M:1) × conversion. In THF (2 mg mL⁻¹) and molecular weights were determined by GPC-LLS (ﬂow rate = 0.5 mL min⁻¹). Universal

calibration was carried out with polystyrene standards, laser light scattering detector data, and concentration detector. Each experiment was

d

e

duplicated to ensure precision. Calculated according to Method A, using the relative integrals of rmr and rmm resonance. The polymerization was

carried out in the presence of benzyl alcohol in a monomer: catalyst: BnOH ratio of 300:1:10.

increased the quantity of monomer, up to 1000 equiv of rac-LA

with 9b as the catalyst in toluene, the polymerization was

completed in less than 10 min, aﬀording the desirable high

molecular weight of 125 kg/mol, with an acceptable

polydispersity index (PDI) of 1.51, as well as a good P_ivalue

of 0.61 at room temperature (Table 1, entry 16). These results

clearly indicate that Ae metal complexes are very eﬀective

catalysts for the synthesis of PLAs with high molecular weight

epimerization.⁸⁵⁻⁸⁷

Despite the high overall rates of polymerization, molecular

weights of the polymers (M_n) agree well with their calculated

values, and increase in linear fashion with an increase in the

quantity of the monomer (Table 1, entries 1−5 and Figure 4),

retaining the narrow molecular weight distributions. A second

set of feed experiments, carried out using the same catalyst,

conﬁrmed that the molecular weights of polymers are under

control (Figure 4). Compared to the barium complex 9b, the

calcium and strontium complexes, 7b and 8b respectively, are

slightly less active (Table 1, entries 1−15), which suggests that

the metal ion plays a vital role in the initiation step for the

activation of the monomer species.

■

Figure 4. Plot of observed PLA M_ntheoand M_nexpi( ) with molecular

weight distribution (PDI) (blue diamonds) as functions of LA:8b in

(25 °C, Tol, 99% conv.) The line indicates calculated M_nvalues based

on the LA:8b ratio.

The catalysts in this family are isoselective. The P_ivalues can

be calculated by substituting integrations of tetrad sequences,

temperature (Table 1, entries 1−3) (Figures FS72 −FS75 in

Supporting Information). To the best of our knowledge, this is

the highest P_ivalue obtained so far by an Ae catalysts.⁷⁰Even

increasing the quantity of rac-LA to 500:1 did not perturb the

degree of isoselectivity (P_i= 0.85) (Table 1, entry 5), which

clearly indicates the controlled manner of polymerization. The

enhancement of isoselectivity may be attributed to the

88

1

from H{¹H} NMR spectroscopy. The tacticity of PLA is

calculated using a set of equations derived from a Bernoullian

statistical model.⁸⁸In the case of the calcium complex 7b,

isoselectivities of P_i= 0.89, 0.87, and 0.85 were achieved when

the stoichiometric ratios of [LA]:[7b] were varied in the order

of 100:1, 200:1, and 300:1, respectively, in toluene at room

E

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Figure 5. (Left) Kinetics plots of the rac-LA versus time in toluene at 25 °C with diﬀerent concentration of catalyst 7b as an catalyst. [7b]₀= 0.5, 1.0,

1.5, 2.0, and 2.5 mM; [rac-LA]₀= 2 M. (Right) Plots of ln k_obsversus ln[7b]₀for the polymerization of rac-LA with catalyst 7b as an catalyst in

toluene at 25 °C. [rac-LA]₀= 2 M.

increased interaction between the lactide and the active metal

site of the catalyst. The high isoselectivities are also maintained

with increase in temperature, as observed from the P_ivalues,

which were quite unchanged (Table 1, entry 19, P_i= 0.86 for

7b). In addition, a high T_mof 186 °C of the polymer (Figure

FS84 in Supporting Information), which is signiﬁcantly higher

than that of the optically pure poly(L-LA), is in good

agreement with the highly isotactic polymer structure.⁸⁹

Additionally, the PLAs obtained from catalyst 7b are stable

up to 280 °C, as shown on the TGA curve (Figures FS81 −

FS83 in Supporting Information), which indicates a very high

level of polymer purity. When complex 8b was used, similar

isoselectivities were achieved for the ROP of rac-LA (Table 1,

entries 6−10) (Figures FS72 −FS75 in Supporting Informa-

tion), but with respect to 7b, the degree of isoselectivity

decreased slightly (P_i= 0.79−0.71) (Figures FS72 −FS75 in

Supporting Information), due to the increase in size of active

metal center from 7b to 8b, which provides a more open

coordination geometry to the metal ion, whereas isoselectivity

is induced due to the steric congestion around the metal center

as we move toward the smaller Ae metals. Subsequently, in case

of complex 9b, the degree of isoselectivity dropped

signiﬁcantly, to 0.58 (Figure FS77 in Supporting Information).

This is quite expected, as a gradual decrease in stereo control

on decreasing steric shielding is typical, and well-reported in the

literature.^70,90

time for a wide range of ln[{Ph₂P(Se)NCH₂C₆H₄N}₂Ca-

(THF)₂] are linear, clearly showing ﬁrst-order dependence on

[rac-LA] for all the ratios (Figure 5). Thus, the rate expression

can be summarized as follows:

−d[LA]/dt = k_app[LA]¹

[{Ph₂P(Se)NCH₂C₆H₄N}₂Ca(THF)₂]^x= k_obs[LA]¹

where k_obs= k_app

[{Ph₂P(Se)NCH₂C₆H₄N}₂Ca(THF)₂]^x

Furthermore, a double-logarithm plot (Figure 5) of the

observed rate constants (k_obs), obtained from the slopes of

the best ﬁt lines to the plots of ln([rac-LA]₀/[rac-LA]_t) versus

time as a function of ln[7b]₀was ﬁt to a straight line

propagation is also ﬁrst-order with catalyst concentration,

indicating that the rac-LA polymerization follows the

monometallic, intramolecular coordination−insertion mecha-

nism (R²= 0.999) with a slope of 0.944. Similar observations

were made about other catalysts 8b and 9b (Figures FS41 −

FS49 in Supporting Information).⁹¹⁻⁹⁴Therefore, the overall

rate equation for the ROP of rac-LA, in the presence of a

catalyst, follows second-order dependence, and may be

represented as

rate = −d[LA]/dt = k_p[catalyst]¹[LA]¹

A comparison of the selectivities listed in Table 1 using the

Bernoullian statistical model shows that a decrease in metal

steric hindrance correlates to increased P_ivalues, while

increasing the steric bulk of the ligands does not result in an

Table 2 represents a comparative study of the rate constants

for the ROP of rac-LA in the presence of catalysts 7b−9b,

which shows that k_obsvalues gradually increase from 0.0139

M⁻¹m⁻¹to 0.029 M⁻¹m⁻¹for 7b and 8b respectively, whereas

the k_obsvalue of 0.036 (M⁻¹m⁻¹) for 9b is relatively higher.

(rac-LA:catalyst = 100:1). On the basis of our previous study,

the enhanced k_obsvalue of the barium complex could be due to

its larger ionic radius, compared to other Ae metal ions,⁷⁰which

enhances the availability of free space around the metal center,

favoring catalyst−LA interactions, which eventually enhances

the ROP of rac-LA. Notably, the calcium and strontium (7b

and 8b respectively) complexes have nearly similar rates, within

error, due to the smaller diﬀerence of ion size between them.

1

appreciable increase in P_ivalues above 0.71. H NMR spectra

revealed the end-group analysis of the polymer chains, which

were capped at one end by one benzyl ester and at the other by

one hydroxyl group (Figure FS69 in Supporting Information).

Kinetic experiments were performed with [rac-LA]₀/[Ae]₀

ratios ranging from 100 to 500. In each case, rac-LA (0.228 g,

2.0 mmol) was added to a solution of each catalyst (7b−9b)

(0.01, 0.02, 0.03, 0.04, 0.05 M respectively) in CDCl₃(1 mL),

at room temperature. The solution was kept in the NMR tube

at 25 °C, and at the indicated time intervals, the tube was

analyzed by ¹H NMR. As expected, plots of [LA]₀/[LA] versus

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constants determined from k_obsvalues divided by temperature

values, as provided in Table TS16 in Supporting Information.

The Eyring plot (Figure 6, right) indicates similar ordered

transition states in a coordination−insertion mechanism.⁸⁹⁻⁹²

ΔG^‡values were calculated for the ROP of rac-LA catalyzed at

25 °C (Table TS16 in Supporting Information).⁹⁷

Table 2. Comparison of Rate Constants for Polymerization

of rac-LA with Various Concentrations of 7b, 8b, and 9b as

Catalyst

s. no.

catalyst

rac-LA: catalyst

k_obsin (M m⁻¹

)

1

7b

8b

9b

7b

8b

9b

7b

8b

9b

7b

8b

9b

7b

8b

9b

100:0.5

100:1

0.0072

0.0201

0.0256

0.0139

0.029

2

In order to understand the mechanism and eﬀect of the

external initiator of the polymerization reaction, we conducted

a typical kinetic study for the ROP reaction of rac-LA in the

presence of an alcohol. This helped us determine the reaction

order with respect to the lactide, catalysts 7b−9b, and benzyl

alcohol (BnOH), as well as establishing the pathway of the

ROP mechanism. The results indicated that the reaction rate

exhibits ﬁrst-order dependence with respect to rac-LA and

catalysts 7b−9b, whereas it follows zero-order dependence with

respect to BnOH (Figures FS65 and FS66 in Supporting

Information). At various concentrations of complex 9b (0.01,

0.02, 0.03, 0.04, 0.05 mM) and concentrations of [rac-LA]₀and

[BnOH]₀ﬁxed at 2 M and 1.0 mM respectively, the k_obsvalues

were found to be identical to those obtained in the absence of

BnOH. In addition, when the concentrations of BnOH (0.02−

0.2 M) were varied, but the concentration of 9b (0.02 M) and

rac-LA (0.228 g, 2.0 mmol) were kept constant, the k_obsvalues

remained unchanged. Thus, the lack of dependence on the

concentration of BnOH conﬁrms its zero-order contribution to

the rate law. The overall rate may be expressed as follows:

3

4

5

100:1

6

100:1

0.036

7

100:1.5

100:2

0.0197

0.039

8

9

0.051

10

11

12

13

14

15

0.0281

0.0489

0.069

100:2

100:2.5

0.0383

0.0575

0.092

Activation Parameters for the ROP of rac-LA. Finally,

we also investigated the eﬀect of temperature on the rate of

ROP of rac-LA using catalysts 7b−9b. From Figure 6, it is clear

that the polymerization rate increased with increase in

temperature. From the ﬁve k_obsvalues determined at diﬀerent

temperatures, the activation energy of the polymerization

reaction using catalysts 7b−9b was deduced, by ﬁtting ln k_obsvs

T⁻¹according to the Arrhenius equation (Figure 6). The

activation energy, E_a, for rac-LA polymerization using 7b was

23.84(3) kJ mol⁻¹. The activation energies for 7b and 8b are

comparable, but they were much lower than that calculated

when 9b was used (Figures FS51 −FS59 and Tables TS12 −

TS16 in Supporting Information).^95,96

The other activation parameters for the ROP of rac-LA

catalyzed by 7b−9b in CDCl₃were found to be ΔH^‡25.87(4)

kJ/mol, ΔS^‡−195.9 (7) J/(mol K), ΔH^‡20.49 (3) kJ/mol,

ΔS^‡−192.1(1) J/(mol K), ΔH^‡22.66(4) kJ/mol, and ΔS^‡

−200.84 (3) respectively (Figures FS51 −FS59 and Tables

TS12 −TS16 in Supporting Information). These values were

calculated from the temperature-dependent second-order rate

−d[LA]/dt = k₂[LA]¹[9b]¹[BnOH]⁰

On the basis of the information set out above, the mechanism is

shown in Scheme 4.

In the ﬁrst step, the phosphorus−selenide double bond (P

Se) of the catalyst is converted into a single bond to form a

zwitterionic species, which initiates the polymerization reaction

in the presence of one molecule of rac-LA (Scheme 4). The

metal−LA interaction in the initiation step is directly

proportional to the size of the metal. The greater the size of

the metal, greater is the possibility of the metal−lactide

interaction period.⁹¹The steric crowding enhances the

initiation of the selenide to a monomer, by releasing the

Figure 6. Arrhenius plot (left) of ln(k_obs) versus (1/T) and Eyring plot (right) of ln(k_obs/T) versus (1/T) for catalyst 9b for the polymerization of

rac-LA (0.01 M) with [LA] = 2.0 M in CDCl₃having E_a= 25.93 (5) kJ mol⁻¹, ΔH^‡= 22.66(4) kJ mol⁻¹ΔS^‡= −200.84 (3) J mol⁻¹K⁻¹(CDCl₃).

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Scheme 4. Proposed Mechanism for rac-LA Polymerization Initiated by Catalyst 7b−9b

unstable energy of repulsion. We have already established

similar results in our work, as reported previously.⁷⁰

In addition, we carried out reactions by loading the monomer

molecular weight of 110 kg/mol, and an acceptable PDI of

1.27. Due to the larger ionic radius of the barium atom, relative

to the calcium and strontium atoms, complex 9b showed the

highest reactivity toward the ROP of ε-CL, similar to what was

observed in previously reported studies using other barium

complexes.^70,78−80According to the literature,⁷⁸⁻⁸⁰a possible

mechanism is proposed in Supporting Information which is

similar to the mechanism proposed for ring opening polymer-

ization of rac-LA. The Ae metal complexes, as precatalysts, were

extremely fast catalysts, enabling almost complete conversion of

1000 equiv of ε-CL in less than 30 s (Table TS21 in Supporting

Information). Therefore, no further kinetic studies were

undertaken for the polymerization reactions of ε-CL.

and catalyst in the ratios of 5:1 and 10:1, and analyzed the

1

reaction mixture using H NMR, ³¹P, and ¹³C spectra to prove

the mechanism pathway. The ¹H NMR spectra clearly indicated

the end group of the polymer, and that the catalyst is bound to

the polymer in the expected ratio (Figure FS64 in Supporting

Information). We also observed two sharp resonance peaks in

the ³¹P{¹H} NMR spectra, at δ 71.9 and 57.6 ppm for 9b

(Figure FS67 in Supporting Information), which indicated the

chemically nonequivalent nature of the two phosphorus atoms

during the polymerization process.

ROP of ε-CL. The ROP of ε-CL was carried out using all the

Ae metal complexes (7b−9b) as catalysts. A catalyst loading of

0.05 mol % in toluene was used as the catalyst (Table TS21 in

Supporting Information) for the ROP of ε-CL. As shown in

entries 1−10 of Table TS21 in Supporting Information, 99%

conversion of the monomer to PCL was achieved within 1 min

when complexes 7b and 8b were used, whereas almost instant

polymerization occurred in the case of complex 9b. In addition,

all the catalysts showed greater control of polymer molecular

weight at levels comparable with M_n(calcd), M_n(GPC), and

narrow PDIs. We then increased the quantity of the monomer,

which showed that the polymerization, up to 1000 equiv of ε-

CL using complex 9b, was completed in less than 5 min in

toluene at room temperature, aﬀording the desirable high

Copolymerization of rac-LA and ε-CL. Achieving high

conversion of both the monomers as well as controlling their

sequence in random distribution have been the key objectives

of the developments dedicated toward the ring-opening

copolymerization of rac-LA and ε-CL. Usually, the considerable

diﬀerence in reactivity between LA and CL in these reactions

tends to generate block-domain microstructures. Therefore,

random copolymers, with 1:1 incorporation of both monomers,

are rare.^63,64The catalytic activity of complexes 7b, 8b, and 9b

in the ring opening copolymerization of rac-LA and ε-CL were

examined in terms of conversion and average block length with

respect to each monomer. Both the barium and strontium

complexes showed good activity for the synthesis of

copolymers, with the activity of the barium complex (9b)

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that copolymers were amorphous in nature and displayed T_gof

−3.3 °C, [T_g(PCL) = −60 °C, T_g(PLA) = 57 °C], (Figure

FS99 in Supporting Information). The thermal behavior

exhibited in the DSC studies supports a structure that is very

close to a random distribution of the two monomers along the

polymer chain.

CONCLUSION

■

In this paper, we have demonstrated the synthesis and

structural study of a series of alkali metal and Ae metal

complexes supported by 2-picolylamino-diphenylphosphane

sulﬁde and selenide ligands. In the lighter Ae metal magnesium,

the selenide ligand behaves in bidentate fashion, leaving both

the selenium atoms uncoordinated. However, in the barium

complex 9b, the selenide ligand exhibits tridentate binding

mode with the metal ion. The heavier Ae metal complexes 7b,

8b, and 9b were utilized for the ROP of rac-LA at room

temperature. Although the ROP reaction is fastest for complex

9b, the highest isoselectivity of PLA (P_i= 0.89) was observed

for the calcium complex 7b, due to the smaller ionic radius of

calcium. A complete kinetic study of the ROP reaction

indicated ﬁrst-order reaction kinetics with respect to the rac-

LA and catalyst concentration. The addition of benzyl alcohol

did not aﬀect the order of the reaction, indicating that Ae

catalysts initiate the ROP of LA in the ﬁrst step. The Ae metal

catalysts 7b−9b are also active for the ROP of ε-CL to yield

polycaprolactone, with a narrow PDI. The strontium and

barium complexes 8b and 9b were used for copolymerization of

rac-LA and ε-CL at room temperature to aﬀord random

copolymers in high yield. The random copolymer formation

using complex 9b was evidenced by the average lengths of the

caproyl and lactidyl sequences (LCL = 1.9 and LLA = 2.7) of

the copolymer, which were formed from a starting feed

composition of CL/LA in the ratio of 1:1. With the success of

being able to perform both homo-polymerization and

copolymerization using a single complex 9b in an eﬃcient

and controlled manner, this system can be extended to the

generation of a variety of biodegradable polymers with very

good control over the polymer properties. Thus, the present

study could lead to a single platform of Ae metal complexes,

resulting in a wide range of biodegradable polymers with high

yield and selectivity.

■

Figure 7. Plot of observed PLA M_ntheoand M_nexpi( ) with molecular

weight distribution (PDI) (blue diamonds) as functions of ε-CL: 8b in

(25 °C, Tol, 99% conv.) The line indicates calculated Mn values based

on the ε-CL:8b ratio.

being comparatively higher than that of the strontium complex

(8b), providing polymers with monomodal molecular weight

distributions [PDI(1) = 1.47; PDI(2) = 1.38]. In the case of the

calcium complex (7b), however, only homo-polymerization

products were formed. In the case of the barium complex (9b),

the incorporation of LA and CL was in the ratio 1:0.18 (85:15)

during the ﬁrst 30 min accounting for a relatively higher

reactivity of LA compared to CL. Upon continuing the reaction

for an additional 2 h, this ratio was found to improve

dramatically to 0.72:1 (42:58), indicating further incorporation

of CL monomers. We also observed moderate levels of CL and

LA incorporation, when the strontium complex (8b) was used,

without the need for an excess CL feed. The ¹³C NMR studies

of the copolymer samples obtained at diﬀerent intervals

revealed the average sequence lengths of caproyl (LCL) and

lactidyl (LLA) to be 1.8 and 3.6 after one-and-a-half hours,

indicating that LA forms a relatively longer chain of PLA with

CL reacting modestly (Table 3, entry 2). However, as the

reaction proceeded further, their values improved signiﬁcantly

to 1.9 and 2.7 after 2 h, indicating the formation of practically

random copolymers (an ideal random copolymer will have LCL

= LLA = 2).^98,99The ¹HNMR analysis of the copolymer

samples obtained after quenching with methanol showed the

presence of methyl ester peaks resulting from transesteriﬁca-

tion, which is considered to lead the randomization of

microstructures during copolymer formation. The thermal

properties of the copolymers were analyzed using the

diﬀerential scanning calorimetry (DSC) curve, which showed

EXPERIMENTAL SECTION

■

General methods. All manipulations involving air- and moisture-

sensitive organometallic compounds were carried out under argon

using the standard Schlenk technique or argon-ﬁlled glovebox.

Hydrocarbon solvents (n-pentane, toluene) were distilled under

nitrogen from LiAlH₄and stored in the glovebox. THF was dried

and deoxygenated by distillation over sodium benzophenone under

argon and then distilled and dried over CaH₂prior to storing in the

a

Table 3. Copolymerization of ε-CL and rac-LA Using Catalyst 9b

b

c

d

catalyst

yield

time (h:m)

CL/LA (mol %)

LCL/LLA

M_ntheo(KDa)

PDI

9b

89

87

85

83

82

00.30

01.30

02.00

02.30

03.30

15/85

28/72

36/64

49/51

58/42

27.9

27.1

26.7

25.9

27.6

27.5

26.3

29.9

26.7

28.5

1.52

1.47

1.37

1.45

1.38

1.8/3.6

1.9/2.7

a

Random copolymerization conditions: 70 °C, 6 h, [monomer]/[Ae] = 400, 0.200 g (1.38 mmol) LA, 0.33 mL (2.63 mmol) CL. M_nand PDI values

b

c

d

1

for all copolymers were determined from the GPC analysis. Isolated yield. Ratio of CL/LA was measured from H NMR. Average CL and LA

e

f

chain length determined by ¹³C NMR. Values obtained by GPC analysis. M_ntheo= ([CL]/[Ae] * %CL * 114.14) + ([LA]/[Ae] * % LA * 144.13).

I

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1

Figure 8. H NMR spectrum of PCL-co-PLA and PLA -co- PCL copolymer by catalyst 9b (Table 3, entry 1).

glovebox. C₆D₆was dried over Na/K, distilled and stored in the

glovebox. ¹H NMR (400 MHz), ¹³C{¹H} NMR (100 MHz), and

³¹P{¹H} (161.9 MHz) spectra were measured on a BRUKER

Hz, 2H) ppm. ¹³C{¹H} NMR (100 MHz, C₆D₆): δ 148.6 (Ar−C),

132.1 (Ar−C), 131.3 (Ar−C), 131.9 (Ar−C), 127.9 (Ar−C), 121.9

(Ar−C), 121.8 (Ar−C) 67.7 (THF), 46.3 (CH2), 25.6 (THF) ppm.

³¹P{¹H} NMR (161.9 MHz, C₆D₆): δ 69.7 ppm. (C₂₆H₃₂Li N₂O₂PSe)

(521.41), calcd C 59.89, H 6.19, N 5.37; found, C 59.61, H 5.92, N

5.01.

AVANCE III-400 spectrometer. Elemental analyses were performed

on a BRUKER EURO EA at the Indian Institute of Technology

Hyderabad. [Li{N(SiMe₃)₂}], [Na{N(SiMe₃)₂}], [K{N(SiMe₃)₂}],

MI₂, CDCl₃, rac-LA, ε-CL were purchased from Sigma Aldrich India.

Preparation of [(Ph₂P(=E)NHCH₂(C₅H₄N)] [E = S (1-H); Se (2-

H)]. In a 100 mL Schlenk ﬂask, 1 equiv of (1.2 mL, 11.4 mmol) of 2-

(aminomethyl)pyridine, 1 equiv of triethylamine (1.56 mL, 11.4

mmol), and 1 equiv of chlorodiphenylphosphine (2 mL, 11.4 mmol)

were mixed together with 40 mL of toluene, and the reaction mixture

was stirred for another 6 h. After that, 1.2 equiv of elemental sulfur or

selenium powder was added onto it, and the mixture was kept under

stirring for another 8 h at 90 °C. Unreacted excess sulfur/selenium was

ﬁltered oﬀ through a G4 frit, and the ﬁltrate was collected. After

evaporation of the solvent from ﬁltrate in vacuo, a colorless solid

residue was obtained. The colorless residue was further puriﬁed by

washing with n-pentane or n-hexane.

Preparation of [{M(THF)_n(Ph₂P(=E)NCH₂(C₅H₄N)}₂] [M = Na, E

= S(4a), Se (4b); M = K, E = Se (5b)]. Inside the glovebox, in a 20

mL sample vial, one equivalent of 1-H or 2-H and 1 equiv of

[NaN(SiMe₃)₂] (25 mg, 0.134 mmol) were mixed together with 5 mL

of THF at room temperature. After 6 h, a small amount of THF (2

mL) and n-pentane (2 mL) was added onto it and kept at −35 °C.

Colorless crystals of 4a or 4b were obtained.

1

4a: Yield: 75 mg (96%). H NMR (400 MHz, C₆D₆): δ 8.30−8.26

(m, 2H, ArH), 8.10−8.06 (m, 8H, ArH), 7.03−7.02 (m, 12H, ArH),

6.87−6.84 (m, 2H, ArH), 6.52−6.48 (m, 4H, ArH), 4.32 (d, J = 8.0

Hz, 4H), 3.49 (br, THF), 1.30 (br, THF) ppm. ¹³C{¹H} NMR (100

MHz, C₆D₆): δ 148.6 (Ar−C), 132.1 (Ar−C), 128.1 (Ar−C), 127.9

(Ar−C), 127.6 (Ar−C), 67.7 (THF), 46.3 (CH₂), 25.6 (THF) ppm.

1-H; Yield: 3.2 g (90%). ¹H NMR (400 MHz, CDCl₃): δ 8.38 (1H,

ArH), 7.95−7.88 (m, 4H, ArH), 7.49−7.48 (m, 1H, ArH), 7.33−7.31

(m, 6H, ArH), 7.11−7.03 (m, 2H, ArH), 4.17 (dd, J = 8.0 Hz, 2H)

4.07 (s, 1H, NH) ppm. ¹³C{¹H} NMR (100 MHz, CDCl₃): δ 156.8

(ArC), 156.7 (ArC), 136.2 (ArC), 134.1 (ArC), 133.2 (ArC), 131.8

(ArC), 128.5 (ArC), 122.3 (ArC), 46.3 (CH2) ppm. ³¹P{¹H} NMR

(161.9 MHz, CDCl₃): δ 59.7 ppm. (C₁₈H₁₇N₂PS) (324.36), calcd C

66.65, H 5.28, N 8.64; found, C 66.31, H 5.09, N 8.49.

^{3 1}P{¹H} NMR (161.9 MHz, C₆D₆):

δ 68.5 ppm.

(C₄₄H₄₈N₄Na₂O₂P₂S₂) (836.9), calcd C 63.14, H 5.78, N 6.69;

found, C 62.83, H 5.54, N 6.45.

4b: Yield: 116 mg (93%). ¹H NMR (400 MHz, C₆D₆): δ 8.08−8.05

(m, 2H, ArH), 8.04−8.02 (m, 8H, ArH), 6.95−6.92 (m, 12H, ArH),

6.90−6.84 (m, 2H, ArH), 6.52−6.48 (m, 4H, ArH), 4.18 (d, J = 9.2

Hz, 4H) ppm. ¹³C-{¹H} NMR (100 MHz, C₆D₆): δ 131.8 (ArC),

130.8 (ArC), 128.4 (ArC), 127.9 (ArC), 127.6 (ArC), 127.1 (ArC),

67.7 (THF), 25.6 (THF) ppm. ³¹P{¹H} NMR (161.9 MHz, C₆D₆): δ

70.4 ppm. (C₄₄H₄₈N₄Na₂O₂P₂Se₂) (930.70), calcd C 56.78, H 5.20, N

6.02; found, C 56.36, H 4.96, N 5.81.

2-H: Yield: 4.1 g (97%). ¹H NMR (400 MHz, CDCl₃): δ 8.40−8.38

(dd, J = 3.9 Hz 1H, ArH), 7.95−7.89 (m, 4H, ArH), 7.52−7.48 (m,

1H, ArH), 7.39−7.30 (m, 6H, ArH), 7.13−7.04 (m, 2H, ArH), 4.17

(dd, J = 8.0 Hz, 2H) 4.03 (s, 1H, NH) ppm. ¹³C-{¹H} NMR (100

MHz, CDCl₃): δ 156.8 (ArC), 156.7 (ArC), 136.2 (ArC), 134.1

(ArC), 133.2 (ArC), 131.8 (ArC), 128.5 (ArC), 122.3 (ArC), 46.3

(CH₂) ppm. ³¹P{¹H} NMR (161.9 MHz, CDCl₃): δ 57.6 ppm.

(C₁₈H₁₇N₂PSe) (371.27), calcd C 58.23, H 4.62, N 7.55; found, C

57.93, H 4.45, N 7.38.

Complex 5b was prepared in an analogous method described for

complexes 4a and 4b by using 1 equiv (50 mg, 0.134 mmol) of 2-H

and 1 equiv of [KN(SiMe₃)₂] (27 mg, 0.134 mmol).

5b: Yield: 135 mg (90%). ¹H NMR (400 MHz, C₆D₆): δ 8.24−8.13

(m, 5H, ArH), 7.02−6.98 (m, 6H, ArH), 6.90−6.86 (m, 1H, ArH),

6.56−6.50 (m, 2H, ArH), 4.26 (d, J = 9.2 Hz, 4H) ppm. ¹³C{¹H}

NMR (100 MHz, C₆D₆): δ 131.8 (ArC), 130.8 (ArC), 128.4 (P

attached o-ArC), 127.9 (P attached p-ArC), 127.6 (ArC), 127.1

(ArC), 67.7 (THF), 25.6 (THF) ppm. ³¹P{¹H} NMR (161.9 MHz,

C₆D₆): δ 70.7 ppm. (C₅₂H₆₄K₂N₄O₄P₂Se₂) (1107.13), calcd C 56.41,

H 5.83, N 5.06; found, C 56.12, H 5.49, N 4.73.

Preparation of [M(THF)₂(Ph₂P(=E)NCH₂(C₅H₄N)] [M = Li, E = S

(3a), Se (3b)]. Inside the glovebox, in a 10 mL sample vial, 1 equiv of

1-H or 2-H and 1 equiv of [LiN(SiMe₃)₂] (23 mg, 0.134 mmol) were

mixed together with 5 mL of THF and kept at room temperature.

After 6 h, a small amount of THF (2 mL) and n-pentane (2 mL) were

added onto it and kept at −35 °C. Colorless crystals of 3a or 3b were

obtained.

Preparation of [κ²-(Ph₂P(=Se)NCH₂(C₅H₄N)Mg(THF)] (6b).

Inside the glovebox, in a 25 mL predried Schlenk ﬂask, compound

2H (50 mg, 0.134 mmol) was mixed with 27 mg of [KN(SiMe₃)₂] and

19 mg of MgI₂(0.0672 mmol) in 10 mL THF solvent at ambient

temperature and the reaction mixture was stirred for 8 h. The white

precipitate of KI was ﬁltered oﬀ, and the ﬁltrate was evaporated under

reduced pressure. The resulting white residue was further puriﬁed by

washing with pentane and was recrystallized from THF−pentane (1:2

ratio) mixture at −35 °C.

1

3a: Yield: 75 mg (96%). H NMR (400 MHz, C₆D₆): δ 8.12−8.03

(m, 5H, ArH), 7.02−7.01 (m, 6H, ArH), 6.87−6.86 (m, 1H, ArH),

6.49−6.48 (m, 2H, ArH), 4.54 (d, J = 8.0 Hz, 2H), 3.59 (m, THF),

1.41 (m, THF) ppm. ¹³C{¹H} NMR (100 MHz, C₆D₆): δ 148.6 (Ar−

C), 132.1 (Ar−C), 128.1 (Ar−C), 127.9 (Ar−C), 127.6 (Ar−C), 67.7

(THF), 46.3 (CH₂), 25.6 (THF) ppm. ³¹P{¹H} NMR (161.9 MHz,

C₆D₆): δ 69.5 ppm. (C₂₆H₃₂LiN₂O₂PS) (474.52), calcd C 65.81, H

6.80, N 5.90; found, C 65.43, H 6.54, N 5.62.

1

Yield: 95 mg (92%). H NMR (400 MHz, C₆D₆): δ 8.39 (m, 2H),

3b: Yield: 68 mg (91%). H NMR (400 MHz, C₆D₆): δ 8.40−8.35

8.32−8.29 (m, 7H, ArH), 7.32 (s, 2H, ArH), 7.32−7.08 (m, 14H,

(m, 1H, ArH), 8.12−8.05 (m, 4H, ArH), 7.16−6.96 (m, 6H, ArH),

6.89−6.79 (m, 1H, ArH), 6.78−6.44 (m, 2H, ArH), 4.57 (d, J = 9.2

ArH), 6.57−6.50 (m, 4H, ArH), 4.44 (d, J = 9 Hz, 4H) ppm. ¹³C{¹H}

J

DOI: 10.1021/acs.inorgchem.7b02847

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

Article

NMR (100 MHz, C₆D₆): δ 148.6 (Ar−C), 132.1 (Ar−C), 131.3 (Ar−

C), 131.9 (Ar−C), 127.9 (Ar−C), 121.9 (Ar−C), 121.8 (Ar−C), 46.5

(CH₂) ppm. ³¹P{¹H} NMR (161.9 MHz, C₆D₆): δ 70.6 ppm.

(C₄₀H₄₀MgN₄OP₂Se₂) (836.94), calcd C 57.40, H 4.82, N 6.69; found,

C 57.17, H 4.52, N 6.46.

ORCID

Hari Pada Nayek: 0000-0003-4247-547X

Tarun K. Panda: 0000-0003-0975-0118

Author Contributions

All the authors contributed equally.

Preparation of [κ³-{(Ph₂P(=Se)NCH₂(C₅H₄N)}₂M(THF)₂] [M =

Ca (7b), Sr (8b), Ba(9b)]. Inside the glovebox, in a 25 mL predried

Schlenk ﬂask, compound 2H was mixed with [KN(SiMe₃)₂] and

alkaline earth metal diiodide (2:2:1 ratio) in 10 mL of THF at ambient

temperature, and the reaction mixture was stirring for 8 h. The white

precipitate of KI was ﬁltered oﬀ, and the ﬁltrate was evaporated under

reduced pressure. The resulting white compound was further puriﬁed

by washing with pentane and was recrystallized from the THF−

pentane (1:2 ratio) mixture at −35 °C.

Notes

The authors declare no competing ﬁnancial interest.

ACKNOWLEDGMENTS

■

This work was supported by the Science and Engineering

Research Board (SERB), Department of Science and

Technology (DST), India, under project no. (EMR/2016/

005150). A. H. and J. B. thank CSIR and UGC India

respectively for their Ph.D. fellowships. We thank Prof. Kazushi

Mashima and Dr. Hayato Tsurugi, Osaka University, Japan, for

their generous support.

1

M = Ca (7b): Yield: (94%). H NMR (400 MHz, C₆D₆): δ 8.18−

8.12 (m, 10H, ArH), 7.01−6.99 (m, 12H, ArH), 6.90−6.88 (m, 2H,

ArH), 6.55−6.49 (m, 4H, ArH), 4.26 (d, J = 8.4 Hz, 4H), 3.60−3.54

(m, THF), 1.42−1.39 (m, THF) ppm. ¹³C{¹H} NMR (100 MHz,

C₆D₆): δ 148.7 (ArC), 136.1 (ArC), 132.2 (ArC), 132.1 (Ar−C),

131.5 (Ar−C) 127.9 (ArC), 122.7 (ArC), 121.9 (ArC), 67.8 (THF),

46.4 (CH₂), 25.7 (THF) ppm. ³¹P{¹H} NMR (161.9 MHz, C₆D₆): δ

71.0 ppm. (C₄₄H₄₈CaN₄O₂P₂Se₂) (924.8), calcd C 57.14, H 5.23, N

6.06; found, C 56.79, H 5.11, N 5.81.

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M = Sr (8b): (91%). ¹H NMR (400 MHz, C₆D₆): δ 8.24−8.18 (d, J

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6.91−6.83 (m, 2H, ArH), 6.58−6.56 (m, 4H, ArH), 4.26 (d, J = 9 Hz,

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AUTHOR INFORMATION

■

Corresponding Author

*E-mail: tpanda@iith.ac.in.

K

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Ring Opening Polymerization and Copolymerization of Cyclic Esters Catalyzed by Group 2 Metal Complexes Supported by Functionalized P-N Ligands

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